In mobile communications, a plurality of paths is generated when a transmitted electric wave propagates through space and the plurality of paths interfere each other at a receiving terminal and the received electric field strength fluctuates from time to time, and this phenomenon is called fading. Commonly, a propagation path in land mobile communications becomes a multi-path propagation path, and so called the fading occurs in which envelope and phase of a received wave fluctuate according to the law of Rayleigh distribution and the law of homogeneous distribution, respectively.
As a technique for alleviating the effects of fading, there is a technology called diversity. This is a technology to reduce fluctuation in propagation by preparing a plurality of receiving branches making use of the phenomenon that fluctuation in propagation varies according to a position in space or a frequency and synthesizing or switching received power in each of the antenna units. For instance, the technology in which a plurality of receiving systems having no correlation concerning the receiving points are provided is called space diversity, the technology in which a plurality of receiving systems having no correlation concerning the frequencies is called frequency diversity, and the technology in which a plurality of receiving systems having no correlation concerning the planes of polarization is called polarization diversity.
On the other hand, as an antenna for a mobile station in mobile communications, omnibearing is required for the horizontal plane so that a receiving level will not change according to the difference in a direction in which an electric wave arrives, also, a beam faces the substantially horizontal direction because an electric wave arrives in the substantially the horizontal direction, that the construction should be rigid, and that the form of the antenna is well harmonized with external appearance of the mobile body. Conventionally there have been used, as representative antennas for mobile stations, a uni-pole antenna based on simple construction, a sleeve antenna little affected by a conductive plate and having excellent directivity, a 5/8 wavelength antenna, a reversed F-shaped antenna which is compact and well suited to incorporation in a portable unit, or the like.
However in the conventional type of antennas for a mobile station, it is impossible to take adequate countermeasures against fading described above, and also in an antenna for a mobile station in mobile communications, it has been desired to introduce the diversity technology as a technology for alleviating effects by fading.